In total knee-replacement (TKR) surgery, there are four broad classes of implants used for resurfacing of the distal femur. In one configuration, the posterior cruciate ligament is retained. In another design the ligament is sacrificed, relying on the articular geometry to provide stability. The third type of device is constrained, in the sense that an actual linkage is used between the femoral and tibial components. According to a fourth arrangement, the posterior cruciate is replaced with a cam on the femoral component and a post on the tibial component.
Many patents have been issued in relation to these design configurations, including the cam-and-post design configuration. Some of the earlier patents in this area include U.S. Pat. No. 4,213,209 to Insall et al; U.S. Pat. No. 4,298,992 to Burstein et al.
Other patents include U.S. Pat. No. 4,888,021 to Forte et al., which teaches a cam-and-post mechanism as well as a linking mechanism. Essentially, each component includes a varying surface and a cam member, so that both the tibial and the femoral component have separate and distinct cams that cooperate with a single tibial post.
U.S. Pat. No. 5,824,100 to Kester et al. discloses a cam/post type of arrangement with a unique type of cam and box enclosure. A portion of the box enclosure is intended to prevent hyperextension and posterior translation. As noted in particular in FIGS. 3 and 4 of the '100 patent, a large space exists between the cam 110 and the post 100 which permits a translation to occur prior to engagement of the cam left of post.
U.S. Pat. No. 5,997,577 to Herrington et al. provides a cam on the femur with a geometry meant to contact the post through a large range of motion. This design attempts to provide the function of multiple cams by providing an area that acts as a separate bearing surface. As such, the cam effectively moves through a range of motion while contacting the post. Depending on the articular geometry which differs than the geometry of the cam post mechanism, this could lead to a variety of problems as well as significantly constrained motion, either between the cam and the post or between the two articulating surfaces.
U.S. Pat. No. 5,658,342 to Draganich et al. describes a cam member with including a bearing surface at complimenting an articulating surface. As in other previous designs, this represents a complex cam geometry meant to capture the post in certain degrees of the range of motion.
U.S. Pat. No. 5,147,405 to Van Zyle et al. Discloses a femoral component with two distinct cam structures, one located at point 44, the other located at 46 in the drawings. The cam member 44 is meant to contact the anterior surface of the post 24 to prevent hyperextension, while cam surface 46 is a posterior located cam meant to have contact throughout the range of flexion. As noted in FIG. 6A of the '405 patent, there is a space between the cam and the post when the knee is in extension, necessitating anterior translation of the femur on the tibia prior to contacting the posterior cam.
Many other patents directed to knee-replacement surgery include cam-and-post mechanisms. But in all cases, either the full range of joint motion is precluded, or translation is allowed to occur which could lead to premature wear. FIG. 1 is a drawing which illustrates a typical prior-art cam-and-post mechanism. Item 102 is a tibial insert or tibial component having a post 103 protruding into a box-like recess of the femoral component 100. FIG. 1A shows the system in extension, whereas FIG. 1B shows the system in flexion. In FIG. 1A, a femoral component 100 includes a cam 101 which has not yet engaged with a post 103.
In FIG. 1B, following a considerable amount of flexion, the cam 101 finally engages with the post 103. Until engagement occurs, however, the component 100 may be permitted to slide relative to the tibial insert. The need remains, therefore, for an improved distal femoral prosthesis having multiple distinct cams contacting a post on its posterior surface to provide a more normal range of motion for cruciate substituting knee replacement.